This model is a one-tenth scale of the MESSENGER space craft.
This model is a one-tenth scale of the MESSENGER space craft. (Algerina Perna, Baltimore Sun)

A mission to Mercury already years past its original expiration date is getting one more month of bonus time to observe the solar system's innermost planet.

The Messenger spacecraft will eventually crash onto Mercury's surface as its orbit gradually lowers, but the mission team at the Johns Hopkins Applied Physics Laboratory in Laurel has delayed that end with bursts of propellant.


They have now devised a way to delay it still further, using the helium gas that pressurizes the propellant tanks as fuel, officials said Monday.

That will give scientists until the end of April, at the latest, to gather high-resolution images of Mercury's dimpled crust, icy polar craters, volcanic flows and tectonic features, they said. They also plan to collect more detailed observations of the planet's magnetic field.

"The team continues to find inventive ways to keep Messenger going, all while providing an unprecedented vantage point for studying Mercury," Stewart Bushman, lead propulsion engineer for the mission, said in a statement.

Messenger launched in 2004 and began orbiting Mercury in 2011 on a one-year mission. But it has been extended twice, and is funded through its crash landing, which had been expected in March.

While the hydrazine fuel that is burned to raise Messenger's altitude is all but used up, the scientists plan to use helium as a propellant once it's gone. The light gas was used to pressurize the fuel tanks and isn't as efficient as the hydrazine because the engines were not designed for it, but it does work, the scientists said.

A burst of the helium gas from the propellant tanks Jan. 21 should raise the orbit from 25 kilometers, or about 15 miles, back up to 80 kilometers, or 50 miles.

The mission has provided unprecedented views of Mercury's surface, including confirming the presence of ice in deep craters near the hot planet's poles.

The spacecraft has observed surprising levels of volcanic activity and "volatile" elements — substances such as sodium and potassium that are abundant on Earth but have boiling points too low to be expected to withstand Mercury's heat.

It also showed that Mercury's magnetic field is asymmetrical, with a magnetic equator about 20 percent of the way toward one pole.

Scientists plan to use the extra time to gather more detailed observations.